Mass spectrometry is an analytical technique that is commonly used to determine the elements that compose a molecule or sample. A mass spectrometer typically comprises a source of ions, a mass separator and a detector. The source of ions may for example be a device which is capable of converting the gaseous, liquid or solid phase of sample molecules into ions, that is, electrically non-neutral charged atoms or molecules. Several ionization techniques are well known in the art, and the particular structure of an ion source device will not be described in any detail in the present specification. Alternatively, the ions to be analyzed by the mass spectrometer may result from the interaction between the sample in its gaseous, liquid or solid phase and an irradiation source, such as a laser, ion or electron beam. The ion emitting sample is in that case considered to be the source of ions.
The ion beam that originates at the ion source is analyzed using a mass analyzer, which is capable of separating, or sorting, the ions according to their mass-to-charge ratio. The ratio is typically expressed as m/z, wherein m is the mass of the analyte in unified atomic mass units, and z is the number of elementary charges carried by the ion. The Lorentz force law and Newton's second law of motion in the non-relativistic case characterize the motion of charged particles in space. Mass spectrometers therefore employ electrical fields and/or magnetic fields in various known combinations in order to separate the ions created by the ion source. An ion having a specific mass-to-charge ratio follows a specific trajectory in the mass-analyzer. As ions of different mass-to-charge ratios follow different trajectories, the composition of the analyte may be determined based on the observed trajectories. By analogy with an optical spectrometer, which allows generation of a spectrum of the different wavelengths comprised in a wave beam, the mass spectrometer allows generation of a spectrum of the different mass-to-charge ratios comprised in a molecule or sample.
In order to detect the ions various known detection devices may be employed at the exit of the mass analyzer. Such detectors can be position sensitive or not, and are well known in the art. Their functioning will not be further explained in the context of the present specification. In general terms, a detector device is capable of measuring the value of an indicator quantity. It provides data for computing the abundances of each ion present in the analyte.
Sector instruments are a specific type of mass analyzing instrument. A sector instrument uses a magnetic field or a combination of an electric and magnetic field to affect the path and/or velocity of the charged particles. In general, the trajectories of ions are bent by their passage through the sector instrument, whereby light and slow ions are deflected more than heavier fast ions. Magnetic sector instruments generally belong to two classes. In scanning sector instruments, the magnetic field is changed, so that only a single type of ion is detectable in a specifically tuned magnetic field. By scanning a range of field strengths, a range of mass-to-charge ratios can be detected sequentially. In non-scanning magnetic sector instruments, a static magnetic field is employed. A range of ions may be detected in parallel and simultaneously.
The resolving power of a mass spectrometer provides a measure of a device's ability to separate two peaks of slightly different mass-to-charge ratios in the resulting mass spectrum. It is defined as R=m/Δm, where m is the mass number of the observed mass and Δm is the difference between two masses that can be separated. The mass separation is translated into the mass dispersion along the detection plane. Δm is determined by measuring the full width at half maximum, FWHM, of the peak corresponding to mass m. The resolving power may not be the same across a range of observed mass ranges.
The Mattauch-Herzog mass spectrometer, as described in J. Mattauch and R. Herzog, Z. Phys., 89, 786 (1934) is a typical high performance wide range parallel mass spectrometric sector-type instrument. As a mass analyzer, the device uses an electrostatic sector followed by a non-scanning magnetic sector. The device provides double focusing of ions on a single straight focal plane at the exit of the magnetic sector, where a range of masses can be detected simultaneously. The principle of double focusing is that ions with different energies and different angles are brought into focus in the same plane. The simultaneous parallel detection improves the detection efficiency and improves the quantitative performance of the device as compared to scanning mass spectrometers. The time dependent fluctuations of the system are eliminated. However, devices using the Mattauch-Herzog geometry normally use a large magnetic sector in order to achieve high performance on a large mass range.
Some variations of the geometry have been proposed as compact mass spectrometers for space exploration, for example in A. O. Nier and J. L. Hayden, Int. J. Mass Spectrom. Ion. Phys., 6, 339 (1971), in M. P. Sinha and M. Wadsworth, Rev. Sci. Instrum. 76, 025103 (2005) or in M. Nishiguchi et al., J. Mass Spectrom. Soc. Jpn, 55, 1 (2006). However, the performance of these designs is limited. The range of mass-to-charge ratios that is detectable in parallel for a single acquisition spans less than ten units, and the mass resolution is limited from tens to a few hundreds.
Patent document GB 1 400 532 A discloses a mass spectrometer device in which a magnetic shunt is arranged downstream of the electrostatic sector and upstream of the magnetic sector.
Patent document U.S. Pat. No. 5,317,151 discloses a miniature sector parallel mass spectrometer. The achieved mass resolution is of 330 FWHM. The achieved mass resolution is reported in M. P. Sinha and M. Wadsworth, Rev. Sci Instrum, 76 025103 (2005), which relates to the same device.
Such known devices are therefore ill-suited for applications where a range of masses from 1 to 35 atomic mass units (amu) at a resolution of at least 1500 is required.
A typical application where such high performance is required lies for example in the area of nitrate pollution detection in surface waters. To date, the N-isotope field still relies on cumbersome sampling and on complex large scale laboratory spectrometers. A portable field mass spectrometer for the analysis of O and H isotopes and for the analysis of 15N and 18O of nitrate would require a mass resolution of at least 1500 in order to eliminate mass interferences, and it would have to be lightweight and robust.